1. Field of the Invention
The present invention relates to an image processing apparatus, a control method of an image processing apparatus, an image forming apparatus, and a storage medium.
2. Description of the Related Art
In recent years, image quality enhancement of an output image and speeding-up of image formation are required more than ever for image forming apparatuses such as printers and copying machines which adopt an electrophotography system, inkjet system, and the like. Especially, in case of a multi-color image forming apparatus of the electrophotography system, a technique using a plurality of photosensitive members corresponding to different colors so as to attain speeding-up is known. Such image forming apparatus corresponds to a tandem type which attains multi-color printing by forming toner images of respective colors on respective photosensitive members, and transferring these toner images in turn from the photosensitive member onto a transfer member or a printing material so as to be superposed on each other.
However, an image forming apparatus often suffers a tilt and curvature of a scanning line due to various causes generated by a printing mechanism. In case of the electrophotography system, a tilt and curvature of a scanning line by a deflection scanning unit are caused by nonuniformity of a lens and a displacement of a mounting position of the deflection scanning unit required to expose a photosensitive member, a displacement of a mounting position of the deflection scanning unit on an image forming apparatus main body. More specifically, a position of an actual scanning line by the deflection scanning unit displaces from its ideal position, that is, misregistration occurs. Especially, in case of a multi-color image forming apparatus which uses a plurality of photosensitive members, a tilt and curvature (misregistration) of a scanning line may be different for respective colors. As a result, when toner images are transferred onto a transfer member or printing material to be superposed on each other, relative positions of these images are displaced, thus causing color misregistration, that is, image quality deterioration.
As a coping method against misregistration of a scanning line and color misregistration caused as a result of the misregistration, a technique of Japanese Patent Laid-Open No. 2003-241131 has been proposed. Japanese Patent Laid-Open No. 2003-241131 has proposed the technique for measuring the magnitude of a tilt of a scanning line using an optical sensor in an assembling process of a deflection scanning device in an image forming apparatus main body, and adjusting the tilt of the scanning line by mechanically adjusting the tilt of the deflection scanning device based on the measurement result.
However, since such mechanical adjustment requires a high-precision adjustment device and movable members, cost may increase, and it is difficult to apply this technique to an inexpensive personal image forming apparatus. In a multi-color image forming apparatus, in recent years, in order to attain a cost reduction, a common deflection scanning device is often used to scan the surfaces of a plurality of photosensitive members corresponding to different colors. In this case, it is difficult for the technique described in Japanese Patent Laid-Open No. 2003-241131 to adjust a scanning line for respective colors.
A method of electrically correcting a tilt and curvature of a scanning line in place of such mechanical adjustment (correction) has been proposed. Japanese Patent Laid-Open No. 2004-170755 has proposed a method of measuring the magnitudes of a tilt and curvature of a scanning line using an optical sensor, correcting bitmap image data to cancel them based on the measurement result, and forming an image using the corrected image data. Since this method electrically corrects a scanning line by processing bitmap image data based on the measurement result, the need for mechanical adjustment members and adjustment processes at the time of assembling can be obviated, thus coping with misregistration of the scanning line at lower cost than the method described in Japanese Patent Laid-Open No. 2003-241131. The misregistration correction by Japanese Patent Laid-Open No. 2004-170755 is divided into correction for one pixel unit and that for less than one pixel. In the correction for one pixel unit, positions of respective pixels of image data are offset in a sub-scanning direction by a correction amount for one pixel unit in accordance with correction amounts of a tilt and curvature of a scanning line. In the correction for less than one pixel, a tone value of each pixel of image data and a pixel value of a pixel which neighbors a pixel of interest in the sub-scanning direction are adjusted. With this correction for less than one pixel, an image corrected by the correction for one pixel unit is smoothed.
However, when the correction based on the method of Japanese Patent Laid-Open No. 2004-170755 is applied to image data of a fine line image including fine lines, a line width of the fine line image to be formed may suffer unevenness. Also, when this correction is applied to image data of a fine image including regular patterns with a high spatial frequency, the fine image to be formed may suffer density unevenness.
(Case of Fine Line Image)
FIGS. 22A to 22D show unevenness of a line width which occurs in a fine line image. FIG. 22A shows image data corresponding to an image including a 1-dot fine line along a scanning direction. FIG. 22A shows tone values of respective pixels by numerical values ranging from 0 to 100%. FIG. 22B shows an example of image data obtained when the correction based on the method of Japanese Patent Laid-Open No. 2004-170755 is applied to the image data shown in FIG. 22A. In general, in an electrophotographic image forming apparatus, a tone value less than one pixel is formed by pulse width modulation (PWM). When an image is formed on a printing material using the corrected image data shown in FIG. 22B, an image shown in FIG. 22C is formed.
In FIGS. 22A to 22D, although the width of the line included in the input image is constant, as shown in FIG. 22A, the width of the line included in the image actually formed on the printing material is uneven in the scanning direction, as shown in FIG. 22C. That is, in the image formed based on the corrected image data, the line width is unwantedly changed for respective positions (scanning positions) p0 to p10 in the scanning direction, and becomes uneven in the scanning direction, as shown in FIG. 22D. This is caused by the nonlinear relationship between the width of pulses generated by PWM and a laser light amount in the electrophotographic image forming apparatus. Furthermore, upon forming a dot having a size not more than one dot, such unevenness is caused by the influence of nonlinear factors during processes of exposure-development-transfer-fixing. For these reasons, tone values of respective pixels in the image data, and actually formed dot sizes and densities do not have a linear relationship, thus forming the line with the uneven width.
When a line is solely included in an image, unevenness of the line width is not so conspicuous. However, when a plurality of lines are included in an image to be repeated at short intervals, a change in line width is visualized as a density change at each scanning position in the scanning direction. When this density change is periodically generated in an image, stripe-like density unevenness becomes conspicuous, resulting in image quality deterioration.
Furthermore, in the electrophotography image forming apparatus, it is difficult to stably form dots especially in an area formed by only dots with a small size like scanning positions p3 to p7 in FIG. 22C. For this reason, the relationship between tone values of image data and dot sizes to be actually formed based on these tone values may be irregularly changed according to the use environment of the image forming apparatus or the number of pages to be printed, and the width of a line included in an image may be irregularly changed.
(Case of Fine Image)
FIGS. 23A to 23D show a case in which image data of a fine image shown as an example in FIG. 23A is corrected in the same manner as in the image data of the fine line image shown in FIGS. 22A to 22D. When the image data of the fine image is corrected, densities are changed for respective positions (scanning positions) p0 to p10 in an image formed based on the corrected image data, as shown in FIG. 23D. This is because dot sizes formed based on the corrected image data become uneven according to the scanning positions. As in the case of the fine line image, since such density change periodically occurs in the image, stripe-like density unevenness becomes conspicuous, resulting in image quality deterioration.
To solve such problems, Japanese Patent Laid-Open No. 2007-279429 has proposed a method for eliminating density unevenness which may occur in an image to be formed by adjusting a correction amount of an image position for a unit less than one pixel based on a measurement value obtained by reading a test pattern image using a sensor.
The method of Japanese Patent Laid-Open No. 2007-279429 suffers the following problems. In general, the characteristics of the electrophotography system as a cause of density unevenness change depending on conditions such as a temperature, humidity, degree of degradation of an image forming device, and the like. For this reason, measurements using a sensor have to be made for different conditions, thus increasing a down time. Also, density unevenness which may occur in an image to be formed may change depending on a pattern of the image. For this reason, various pattern images have to be formed, and measurements have to be done for the respective formed pattern images, thus increasing a consumption amount of toner used to form the pattern images in addition to an increase in down time. Furthermore, density unevenness which may occur in an image to be formed appears as very small density changes. In order to measure such very small density changes, a high-precision sensor is required, resulting in an increase in cost.